Workshop Report 24

Modified and enhanced biodegradability testing

Kees van Ginkel

Akzo Nobel, the Netherlands

Ready biodegradability tests only detect growth-linked biodegradation whereas simulation tests also measure cometabolic transformations. Growth-linked biodegradation is superior compared to cometabolic degradation. Ready biodegradability test results should therefore be treated with priority when assessing the biodegradation potential. Modifications and enhancements of the ready biodegradability tests have been designed to improve biodegradability assessments with the methodology of ready biodegradability tests.

This presentation focused on methods of modified and enhanced biodegradability testing. A justifiable outcome of ready biodegradability tests with poorly water soluble and toxic substances requires methods preventing inhibitory effects due to high initial concentrations and/or limited bioavailability. A few examples were discussed. Decrease of the concentration of quaternary ammonium salts in the water phase to a non-toxic level can be achieved through the addition of silica gel, humic and lignosulphonic acids. Silicone oil was introduced into the test vessels when testing with a fragrance to prevent toxic effect and loss of the volatile test substance. Introducing agitation of the test media and lower initial test substance concentrations resulted in a ready biodegradability results with dialkylamines. Biodegradation of poorly water soluble substances though an increase of the bioavailability with surfactants has also been demonstrated.

Competent microorganisms present at low numbers in the environment are often not detected due to the low inoculum size of ready biodegradability tests. This may be solved through acclimatisation at low test substance concentrations. N-methylpiperazine is not biodegradable in ready biodegradability tests nor is it in most inherent biodegradability tests. However, a prolonged Closed Bottle test result indicated that microorganisms capable of utilising N-methylpiperazine as growth substrate do exist. Inocula for Closed Bottle tests with increased number of competent micro-organisms were obtained through acclimatisation of activated sludge and micro-organisms present in river water at low concentrations (1 µg/L to 1 mg/L). The modifications and enhancements improved the assessments of the biodegradation potential with the methodology of ready biodegradability tests. Microbial growth results by definition in adaptation of microbial communities and increased degradation rates in (eco)systems. Detection of growth-linked biodegradation with modified and enhanced tests should, therefore be more appreciated than simulation test results. The inability to detect growth-linked biodegradation with standard ready biodegradability tests are often caused by the high test substance concentrations not occurring in the environment and small inoculum sizes. Enhancements like extending the duration of ready biodegradability tests and low level adaptation do improve detection of growth-linked biodegradation. Detection of growth-linked biodegradation with modified and enhanced tests should also be more appreciated than simulation test results.

The talk concluded that ‘the assessment of biodegradation should become more science based’.